The invention relates to a method and a measuring apparatus for determining the exposure time in photographic copying by means of enlarging or copying apparatuses.
For determining the required exposure time for a given original to be copied, a light measuring apparatus, also called photometer, is normally used, by means of which the original is measured with respect to its density in order to derive from it the exposure time according to the known relation EQU I.sub.E .times.t.sub.exp =K (1);
wherein I.sub.E is the intensity of the copying light, which penetrates the original to be copied; t.sub.exp is the exposure time, i.e., the duration of the effect of the light intensity I.sub.E on the copying material; and K is a constant, the value of which must be determined during a calibrating phase with respect to a test original and the used copying material.
Light measuring apparatuses of this type generally comprise a measuring probe, which can be positioned in the copying plane of the enlarging apparatus, for measuring the light intensity at a selected point of the projected picture, and of a signal evaluating circuit for converting the measuring signal to a corresponding value of the exposure time. The thus determined exposure time is indicated on a display device or the corresponding time signal is used directly for controlling the exposure in the enlarging apparatus. Light measuring apparatuses are also known, which have storage devices for the temporary storing of the measuring values with respect to various measuring points, and which have computing devices for carrying out computing operations by means of the stored measuring values, such as the formation of a mean value or the determination of the difference between the extreme values. A light measuring apparatus of this type is described, for example, in German Patent DE-C 2935705, where the exposure time can be derived, instead of from an individual value, from the mean value of several measurements at different points of the projected picture.
Applications exist, in which it is preferred to expose by means of a fast given exposure time and to control the exposure, instead of by means of the time, by means of the intensity of the light, for example, by means of a diaphragm. The exposure with a constant exposure time (approximately equal to that of the calibrating conditions) is used, for example, in order to prevent the occurrence of the known Schwarzschild Effect (deviation from the reciprocity theorem), which results in anomalies in the exposure if the exposure time varies within an excessive range. Similarly, if the light measuring apparatus does not control the exposure directly, but the determined time must first be transmitted to the control device, it may be advantageous for practical reasons and in order to reduce the risk of errors, to work with a constant exposure time and to adjust the diaphragm correspondingly.
The objective in the indicated cases is therefore to convert a value of the exposure time, which was determined by means of a light measuring apparatus, into another given value and to change the light conditions correspondingly. This will not result in any problems if the indicated exposure time is the result of an individual measurement at an individual point of the picture, because then a direct relationship exists between the change of the light intensity and the indicated exposure time. However, the conversion by means of the known light measuring apparatuses will no longer be possible if the indicated exposure time is the result of a formation of a mean value from the measurement of a plurality of selected points of the picture.